My website does very well on other search engines, like Yahoo and MSN, but it's nowhere to be found on Google. Am I being penalised?
Firstly, it is important to understand that all the search engines develop their own algorithms, which are constantly being updated and tweaked in an attempt to improve the results they offer. Ranking highly on Yahoo for a given phrase is no guarantee that the website will achieve the same or even similar results on other search engines. Similarly, if a website does manage to achieve a top result for a given search on any search engine, it would be short sighted indeed to assume that it will remain highly ranked for that search term. The changes in the algorithms can cause dramatic changes in the SERP's (Search Engine Results Pages).
Sites don't get banned or penalised unless something has been done in an attempt to deceive or manipulate the natural search results. Typical attempts at deception include hiding content on the page and/or keyword stuffing, or attempts to artificially inflate link popularity. Rest assured, if you don't make any attempt to deceive the search engines, or fool them into giving you a higher rank for a given phrase, then you have no need to worry about penalties.
Given the constantly shifting algorithms, and constant influx of new and updated page content on other sites, the best advice is to optimise your site for lots of related phrases. This improves the sites chances of ranking highly for a selection of search terms, not just one. Also, the search engines will respond favourably to seeing lots of related phrases and terms. Any quality content page that discusses a subject will include a good selection of terms related to that subject. For example, if Google is attempting to determine the subject of one of your content pages, it will be better able to do so if it detects a selection of common phrases that are frequently used when talking about that subject.
This principle is commonly referred to as either Latent Semantic Analysis (LSA), or Latent Semantic Indexing (LSI).